PET quantitative analysis techniques seek to quantitatively assess the tissue radioactivity concentration, typically scaled by the injected activity per unit mass or another normalization factor. The quantitative analysis is based upon a linear relationship of patient image intensity with uptake of the imaging agent. For a fluoride-18 (F18) radioisotope, the most common quantitative image analysis metric is Standardized Uptake Value (SUV), which is calculated either pixel-wise yielding a parametric image, or over a Region of Interest (ROI). However, the linear relationship used to transform image intensity to tissue radioactivity concentration is derived from a SUV calibration which is typically computed as a single curve for the entire system, i.e. the same SUV calibration curve is used for each detector pixel. The SUV calibration curve also incorporates pixel dead time. During clinical imaging the PET system typically operates near or in the so-called “paralyzed detector” regime, in which detector dead time is a significant factor. This dead time results because there is minimum time between gamma particle detection events—that is, if two gamma events impinge on the detector in (too) short succession, then the second event will not be detected because the detector has not yet reset after detecting the first event. Because a single system-level SUV calibration curve is used, the dead time is assumed to be the same for all pixels. In the SUV calibration curve, the dead time is seen as a sub-linearity to the singles rate-versus-radioactivity curve due to reduced observed counts at high radioactivity level caused by “missed” counts during the dead time.
SUV calibration typically employs a cylinder source which contains F18 at a high activity level. The calibration source is located at the gantry ISO center, and parallel to patient bed (i.e. cylinder axis oriented along the axial direction). PET data acquisition is performed periodically, until the source is decayed to a level below detection. The reason for placing the cylinder source at the ISO centre is to factor out the variations caused by positioning and source unevenness. As the radioactivity concentration of the calibration source is known as a function of time throughout the decay process, the result is the desired curve relating image intensity to radioactivity level. This process is known as SUV calibration.
With reference to FIG. 1, the difference between SUV calibration and patient scan is illustrated. The SUV calibration uses a uniform cylinder phantom, with detectors at locations A, B, and C getting the same amount of exposure and having the same singles count rate. However, during a patient scan, detectors at A and B receive more exposure than the detectors at C, thus their singles rates are different.
However, such detector pixel-level effects are not accounted for by the single system-level SUV calibration curve.